Tag Archives: trace fossil

Skolithos is a common type of trace fossil that has been found in rocks as old as 541 million years. Trace fossils are not the fossilized remains of organisms but rather the burrows, footprints, and other structures that resulted from the animal’s activities.

In the case of skolithos, it’s widely believed that a vermiform (resembling a worm) animal created the straight, vertical, tube structures. These worm-like critters probably lived by filtering plankton from the turbulent water of a shallow marine environment. The vertical tubes may have been a dwelling place to retreat to, though their specific purpose is not known.

In the above picture, captured in north central Arkansas, a sandstone has weathered to reveal skolithos traces permeating the approximately 460 million year old rock. This example is from an exposure of the St. Peter Formation, Buffalo National River Park, Marion County, Arkansas.

Rosselia – or Rosselia socialis – is a trace fossil that’s common to rocks deposited in a variety of shallow marine environments such as estuaries, tidal flats, lagoons, etc.. This picture was taken in a quarry in the Pennsylvanian Bloyd Formation, near Greers Ferry, Arkansas. Rosselia is a funnel-shaped burrow with concentric cone-like layers, and a sandy plug near the center. The picture shows a side view, or cross-section, of several burrows.

Like many trace fossils, Rosselia was made by a soft-bodied animal that was rarely if ever fossilized. We only know it existed because of the burrows it left behind. They may represent places an animal lived, fed, or perhaps both.

One theory suggests the burrows were occupied by worm-like animals that fed by filtering nutrients from sediment, then excreting the sediment outward around their bodies in concentric muddy layers. When new beds of sand were deposited, the animal would crawl to the top of the sand bed and make a new burrow; this behavior is clearly evident in burrows at the center of the photo.

Above is a commonly misunderstood geologic feature called an Asterosoma. Almost everyone, upon first seeing an Asterosoma, thinks it’s either a fossilized plant, flower, or some kind of fossilized animal – usually an octopus.

Asterosomas are actually trace fossils left behind by ancient marine animals (most likely worms or shrimp) that burrowed through mud in a delta or tidal-flat. This one was found in the Carboniferous section of north Arkansas and is roughly 300 million years old. These trace fossils are called Asterosoma because of their star-like shape.

In cross-sectional view, multiple Asterosomas sometimes overlie one another connected by a central vertical tube – like a garland of Asterosomas. This suggests that, as new sediment was periodically washed into the environment, the animal may have burrowed its way back to the top of the mud and wallowed out another home for itself. The animal itself was too soft-bodied to be preserved in the rock record.

This week was a hot one, so we did some creek work, but also did some road work. Went up a section of lower Lost Creek on Monday. Narrowed down the Cane Hill/Imo to a fairly small area.

Saw more nice trace fossils in the Imo.

Got a few points on the Middle Fork where it leaves the Shirley quad and enters Greers Ferry Lake.

Also, took points on a 4 different shale pits or quarries.

The persimmons aren’t quite ripe yet!

Found these deformation bands right by Highway 16 near Fairfield Bay. These bands are associated with structural features like faults and folds. They form in more coarsely grained sandstone when the stress crushes the individual sand grains along a plane, then recrystallizes to form a slightly more resistant lithology, that when weatherd, stand in slight relief. We will keep looking in this area for more clues and hopefully figure out what the rocks are trying to tell us!

We’re still working on the Shirley quad this week. Started out Monday by dropping down a steep side drainage off Nubbin Ridge nearly to the Middle Fork, then right back up another one. Got good points at the base of the Witts Springs and Cane Hill. The Imo is at the bottom of the valley but the very large boulders eroding at the top cover most of it. Relief was around 400 feet.

Typical Witts Springs on Lost Creek

near Cane Hill top (1)

Cane Hill (2)

Cane Hill (3)

Cane Hill (4)

Cane Hill (5)

Cane Hill (6)

Cane Hill (7)

near Cane Hill bottom (8)

near top of Imo

Tuesday and Wednesday we were back in the headwaters of Lost Creek. There are many branches to this creek, and there seems to be a fairly large structure crossing them approximately east/west. Could be a fault, or maybe just a fold of some kind—will take much more data collection to be sure. We did observe a repeating sequence from higher elevations to lower—now we just have to figure out which formation they are in! In the Ozarks, the Pennsylvanian-aged rocks are a series of sandstone and shale units deposited in a shallow marine environment. That means they are all very similar to each other and the differences are subtle and vary from place to place. To differentiate one formation from another, we depend a lot on context, which means we must take meticulous notes on each outcrop in the hopes we can see a pattern in the stratigraphy that matches something known. We’re starting in the northwest corner of Shirley because we’ve already mapped the surrounding quads in that area, so have a pretty good idea of the geology there. Things are getting more complicated as we reach the structure because the strata are displaced somewhat from one side to the other. We will continue to add points next week in order to discover what the rocks are doing there.

ripple beds

Since I’ve mentioned some of the formations in the area and how difficult they are to differentiate, let me discuss one that has stayed fairly consistent across the Boston Mountains Plateau. The Cane Hill is a Member of the Hale Formation, and its type section is in northwest Arkansas. It is typically very thin- to thin-bedded, ripple-bedded, very fine-grained sandstone with interbedded shale and thicker shale units. There are abundant trace fossils in the Cane Hill including asterosoma which is a somewhat star-shaped feeding burrow.

Trace Fossils:

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Asterosoma:

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Bluffs of this sandstone often affected by a particular weathering phenomenon where the cement holding the sand together actually solutions out which lets the individual grains fall away eventually forming a concave structure. I have included a good set of photos of the Cane Hill and a few of its characteristic structures. It is getting to be over 300 feet thick as we continue to map toward the southeast.